This is an enormous post but I'm hoping someone with experience may be able to help:

I adopted Martha and Madison about five years ago. Martha was about a year old and had just been spayed and Madison was about six months. When we adopted them we were led to believe that Madison was a girl, but, surprise, we took her to the vet and she was a he who we renamed Bruce. We had him neutered the next month. For about three years they got along totally fine and then last year Martha started attacking Bruce. Blood has never been drawn but there is a lot of low growling, hissing, posturing and eventually they jump at each other. We are usually able to break them up and separate them. I have kept them apart for week long stretches and then reintroduced them to each other about three times. They will be fine for about a month or two and then the fighting starts again. I have taken the cats to the vet twice on the matter and each time I get the "we don't know much about cat behavior, cats will fight" speech.

Last week we had a terrifying incident. The cats had not fought for at least two months. I was doing the dishes and my husband was watching T.V. when all of a sudden the cats ran into the living room hissing and screaming. Martha pounced on Bruce and she was absolutely relentless, biting his neck and digging her claws into his back. My husband threw cushions at them, I sprayed water but she would not get off him. After about a minute of this, (which felt like an eternity), Martha eventually relented but Bruce was foaming at the mouth. We swooped up Martha and put her in our den in order to separate the cats. We tried to approach Bruce but he kept hissing so we gave him his space. After about a minute, he got up and drank some water. After about five minutes he seemed his old self. We took him to the vet the next day and they said he was fine but both my husband and I are completely shaken by the event, I was honestly surprised that Martha was not able to draw blood.

Since this incident we have kept the cats completely separated. We have also taken Martha to the vet where they said she was fine. My vet seemed genuinely perplexed by the incident and could offer no behavioral advice.

I have scoured the internet and the library for any information that might help and all I found was this from about.com:

It describes a type of female cat aggression where she turns on her son as a sort of 'get out of the nest' aggression. The site goes on to say that this is common in lions, "where a pride of lions will chase off the adolescent males, forcing them to move on elsewhere, to establish their own prides". This leads me to believe that there may be no hope.

Another strange tidbit, Bruce still suckles. Although Martha is not producing milk he still will suck at her tit! The vet told us that we did not need to ween him and that she would just push him away when she felt like it but this never happened.

I love my cats more than anything else in the world, (don't tell my husband ) but I am at my wits end. Bruce is a very fragile sort of cat. He has never stood up for himself and I fear that he is going to get seriously hurt. Has ANYONE had experience with this sort of thing? Do I need to find a good home for one of them, heaven forbid, or is this a salvageable situation?